1. Field of the Invention
The present invention relates to a combustion system for direct fuel injection diesel engines, and more particularly, wherein a combustion chamber bored into a head of a piston is shaped like a deep dish of a small opening ratio and a fuel injection nozzle is provided with upper and lower rows of nozzle holes, thereby synthetically and simultaneously attaining both reduction of NOx and black smoke in exhaust gas and reduction of fuel expenses.
2. Background Art
Conventional direct fuel injection diesel engines generally have shallow dish combustion chambers formed in the heads of pistons. Such a combustion chamber has a great opening ratio because its peripheral edge is considerably close to the outer periphery end of the piston. Therefore, the travel of fuel from nozzle holes of a fuel nozzle facing the combustion chamber to the side wall of the combustion chamber becomes large so that the injected fuel is completely sprayed with almost no air stream, thereby increasing the coefficient of air-utilization, then burnt.
Furthermore, Japanese Utility Model Sho. 61-29940, for example, discloses a fuel injection nozzle provided with upper and lower nozzle holes in zigzags arranged radially around the axis of the fuel injection nozzle. In this art, the combustion chamber is vertically stepped so that fuel is injected from the upper nozzle holes toward the wall of the upper combustion chamber, and from the lower nozzle holes toward the wall of the lower combustion chamber, respectively.
However, the above art is only intended to increase the coefficient of air-utilization for improvement in fuel efficiency. The fuel spray travel from the upper nozzle holes to the wall of the upper combustion chamber is substantially as large as that the lower nozzle holes to the wall of the lower combustion chamber for an attempt of even combustion.
Japanese Utility Model Sho. 43-20403 discloses the lower nozzle holes are reduced in the diameters thereof and the number thereof. This document of the conventional art does not disclose a combustion chamber of a piston. However, the angle of the lower nozzle hole with the axis of the fuel injection nozzle (the nozzle hole angle xcex2 in the document) is comparatively large. Also, there is an account that the complete fuel combustion, which cannot be compensated only by improvement of air stream, is attained by distribution of fuel injection in consideration of the shape of combustion chamber at a final period of compression. Thus, presumably, the target of fuel sprayed from at least the lower nozzle holes is the edge of outer wall of the combustion chamber so as to supply the fuel spray into the piston combustion chamber even at the final period of compression. Perhaps, the unshown bottom central portion of the combustion chamber projects upwardly so as not to be provided for combustion. However, if the bottom of the combustion chamber is flat, it may be to say that air is insufficiently utilized around the central portion of the chamber bottom.
A deep dish combustion chamber is disadvantageous in that the air in the peripheral portion of the piston is not efficiently utilized. Even if the conventional fuel injection nozzle having upper and lower rows of nozzle holes is used, the air about the center portion of the combustion chamber is not effectively utilized because the lower nozzle holes to the side wall of the combustion chamber for fuel injection. If the air in such portions is sufficiently utilized, the combustion efficiency may be improved to reduce fuel expenses.
In a direct injection diesel engine according to the present invention, for the purpose of simultaneous attainment of the two antinomic requirements, that is, reduction of fuel expenses and reduction of NOx, black smoke and the like in exhaust gas (exhaust emission), a piston is formed at its head with a deep dish combustion chamber of a small opening ratio. A fuel injection nozzle is provided with a pair of zigzagged upper and lower rows of nozzle holes. The direction of the nozzle holes in the upper row is set so that fuel injected therefrom reaches a side wall of the combustion chamber. A ratio of a fuel spray travel therefrom to a diameter of each of the nozzle holes in the upper row is set to 150-250. The direction of the nozzle holes in the lower row is set so that fuel injected therefrom impinges upon a bottom surface of the combustion chamber. A ratio of a fuel spray travel therefrom to a diameter of each of the nozzle holes in the lower row is set to 100-180. A ratio of a total area of all the nozzle holes in the lower row to that of all the nozzle holes in both the upper and lower rows is set to 0.25-0.35. The number of the nozzle holes in the upper row is set twice as large as that of the nozzle holes in the lower row.
Since fuel is injected from the fuel injection nozzle formed with such nozzle holes to the above-mentioned deep dish combustion chamber, 65-75% of fuel injected from the total area of all the nozzle holes is burnt in the vicinity of the vertical wall of the combustion chamber with a slight swirl, spurt and reverse spurt. The remaining 25-35% injected fuel directly impinges upon the wall (bottom surface) to be spread into minute particles and burnt. The fuel from the nozzle holes in the upper row is spread while moving up the vertical wall so as to be burnt with the air along the periphery of the piston. On the other hand, the fuel from the nozzle holes in the lower row is also spread to the vicinity of the central portion of the combustion chamber because of its impingement against the bottom surface. Therefore, the air in the whole combustion chamber is effectively used so as to improve efficiency of combustion and reduce fuel expenses.
Thus, according to the combustion system for direct injection diesel engines of the present invention, two kinds of combustion are simultaneously performed in one combustion chamber, thereby synthetically and simultaneously attaining both reduction of exhaust emission and reduction of fuel expenses.
The above-mentioned fuel injection nozzle is further constructed such that both extension lines of axes of any adjacent two of the nozzle holes in the upper row form the same angle on the axis of the fuel injection nozzle, and both extension lines of axes of any adjacent two of the nozzle holes in the lower row form the same angle on the axis of the fuel injection nozzle. Therefore, when viewed in plan, fuel is equally injected from all the nozzle holes in the upper row and from all the nozzle holes in the lower row respectively. The combustions by sprays from all the nozzle holes in the respective upper and lower rows are equalized, thereby improving efficiency of combustion and reducing fuel expenses.